It has been long understood that the human ear perception of loudness varies with frequency when the sound pressure level is held constant. Although recording engineers may attempt to compensate for the non-flat response of the human ear-brain system, that compensation is valid only when the music is played back at the same sound pressure level at which the compensation is applied. In case of live music where no compensation is applied, the desired playback attempts to reconstruct the same sound experience as the live performance. For these reasons, when compensating for loudness perception differences, it is not necessary to compensate for the non-flatness of the ear-brain system, but rather to compensate for the differences in playback levels. Other considerations include physical limitations of audio systems and compensation systems, the applicability of loudness measurements to the media to be played, and listener preferences.
Many researchers and engineers have contributed to the understanding of perceived loudness with respect to frequency variations and have derived perceived loudness versus frequency curves (Stevens, S. S., Perceived Level of Noise by Mark VII and Decibels (E), Journal of the Acoustical Society of America, Vol. 51, pp. 575–601, 1972). Further work added to the body knowledge by generating a set of differential loudness curves of various playback sound pressure levels for each recording sound pressure level (Holman, T and Kampmann, F, Loudness Compensation: Use and Abuse, Journal of the Audio Engineering Society, Vol. 26, No. 7/8, July/August 1978). These curves may vary in gain and shape depending on the recording levels and playback levels. Although such understanding between recording and playback is desirable, it is not directly applicable because the sound pressure level of the music at the time of recording is typically not known. What is recorded is not the actual sound pressure level, but some electrical, mechanical or numerical representation of the sound pressure level, where the mapping between the sound pressure level and the recorded representation is not preserved. Therefore, conventional compensation systems use a general set of loudness curves and some mapping to the sensed recorded level or to the system volume control itself to approximate this functionality for all input music.